1. Field of the Invention
The apparatus of this invention resides in the area of blast machines and more particularly relates to a blast machine which propels shot at metal sheets or plates in order to clean them so that they are in suitable condition for further processing.
2. Description of the Prior Art
Blast machines have been used in the prior art to clean sheets, strips and plates of metal, some of which may initially be provided in the form of rolls or coils, for cleaning and descaling by delivering such workpieces on conveyors through long chambers which have shot blast wheels disposed above and below the sheet metal being cleaned. Such prior art machines have provided a plurality of shot blast wheels disposed in such long chamber through which the sheet is passed wherein each shot blast wheels cleans a specific portion of the upper or lower surface of the sheet. Some of the prior art cleaning machines have blast wheels disposed at a 45 degree angle to the metal sheets being cleaned.
When cleaning metal sheets, several problems arise in the prior art. One of these problems is warpage when cleaning thin-gauge metal sheets. Another problem is excessive shot buildup on the top of the metal sheet causing inadequate cleaning or shot blinding by preventing the top blast wheels from adequately cleaning the top of the metal sheet. Since there is already shot resting on the metal sheet, this buildup of shot blocks the impact of the shot stream coming from the blast wheels.
Other difficulties with the prior art stem from cleaning problems in the blast chambers themselves due to shot rebounding into the oncoming particle streams propelled by the shot blast wheels. Furthermore, excessive shot blast wheel wear can be caused by shot rebounding back into the wheel. ln addition, high chamber wear can occur from shot which misses the metal sheet and hits the inside walls of the blast chamber at a direct 90 degree angle. These problems with prior art blast machines are compounded when deformation and warping of metal sheets is caused by blast machines that have hot spots in their blast patterns causing unequal blast intensity over the surface of the metal sheet resulting in deformation of the metal sheet being cleaned.